Depth Of Interaction Measurement Research Articles

Depth of Interaction Measurements Based on Rectangular Light Sharing Window Technology and Nine-Crystals-to-One-SiPM Coupling Method

Depth of interaction (DOI) decoding capability shows great application potential for positron emission tomography (PET), especially for high-resolution small-animal PET or long axial field of view PET systems. On the basis of GATE optical simulation results, we presented a novel $9\times 9$ lutetium–yttrium oxyorthosilicate array with 10-mm rectangular light-sharing windows (RLSWs) and assembly the detector with nine crystals coupled to one silicon photomultiplier (SiPM) method; this caused DOI-related position shifts in the crystal decoding spots of the flood map. Homogeneous radiation was used to establish transfer functions to convert spot shifts into DOI measurements. The conventional collimated radiation experiments were used to assess the accuracy of the DOI measurements. Flood map results show that 81 segmented crystals can be clearly resolved, including 16 crystals adjacent to two RLSW windows (M-type), 40 crystals adjacent to one RLSW window (E-type), and 25 crystals not adjacent to any RLSW window (C-type). The mean absolute error of all DOI-capable crystals, M-type crystals, and E-type crystals are 2.15 ± 0.61, 1.95 ± 0.50, and 2.23 ± 0.63 mm, respectively. The novel nine-crystals-to-one-SiPM coupling technology is a low cost approach for constructing high-performance DOI detector modules. Detectors using nine-crystals-to-one-SiPM coupling technology are good candidates for high-resolution preclinical PET and for total-body PET.

Position and depth of interaction measurements using silicon photostrip detectors and CsI(Tl) crystal

The photoelectric absorption of gamma rays in silicon occurs with such low probability that scintillation material is employed in the photostrip detector. We fabricate single-sided photostrip sensors which are sensitive to visible light. Two photodetectors in a photon counting mode are optically coupled with the scintillator with a sandwich structure. The photostrip sensors have 128 strips, a 200μm strip pitch and a size of 3.8×2.8cm2. Sensor signals are read by a 128-channel VA1TA3 readout chip. A CsI(Tl) crystal is then optically combined to measure both of depths of interaction (DOI) and 2-D images, while the position of the LED are varied. We also performed a simulation of gamma ray irradiation at different locations inside the scintillator using the GEANT4 Monte Carlo simulation tool. We measured the 2-D position of incoming light with two photostrip sensors, and also measured the DOI and 2-D position of scintillating light by placing a scintillator between the photostrip sensors. The DOI measurement results and the GEANT4 simulation results were compared.

Programmable integrated front-end for SiPM/PMT PET detectors with continuous scintillating crystal

AMIC architecture has been introduced in previous works in order to provide a generic and expandable solution for implementing large number of outputs SiPM array/PMT detectors. The underlying idea in AMIC architecture is to calculate the moments of the detected light distribution in an analog fashion. These moments provide information about energy, x/y position, etc. of the light distribution of the detected event. Moreover this means that a small set of signals contains most of the information of the event, thus reducing the number of channels to be acquired.This paper introduces a new front-end device AMIC2GR which implements the AMIC architecture improving the features of the former integrated devices. Higher bandwidth and filtering coefficient precision along with a lower noise allow to apply some detector enhancements. Inhomogeneity among detector elements throughout the array can be reduced. Depth of interaction measurements can be obtained from the light distribution analysis. Also a common trigger signal can be obtained for the whole detector array. Finally AMIC2GR preamplifier stage close to SiPM output signals optimizes signal to noise ratio, which allows to reduce SiPM gain by using lower operating voltages thus reducing dark noise.

Maximum likelihood positioning for gamma-ray imaging detectors with depth of interaction measurement

The center of gravity algorithm leads to strong artifacts for gamma-ray imaging detectors that are based on monolithic scintillation crystals and position sensitive photo-detectors. This is a consequence of using the centroids as position estimates. The fact that charge division circuits can also be used to compute the standard deviation of the scintillation light distribution opens a way out of this drawback. We studied the feasibility of maximum likelihood estimation for computing the true gamma-ray photo-conversion position from the centroids and the standard deviation of the light distribution. The method was evaluated on a test detector that consists of the position sensitive photomultiplier tube H8500 and a monolithic LSO crystal ( 42 mm × 42 mm × 10 mm ). Spatial resolution was measured for the centroids and the maximum likelihood estimates. The results suggest that the maximum likelihood positioning is feasible and partially removes the strong artifacts of the center of gravity algorithm.

Front-end circuit for position sensitive silicon and vacuum tube photomultipliers with gain control and depth of interaction measurement

Silicon Photomultipliers, though still under development for mass production, may be an alternative to traditional Vacuum Photomultipliers Tubes (VPMT). As a consequence, electronic front-ends initially designed for VPMT will need to be modified. In this simulation, an improved architecture is presented which is able to obtain impact position and depth of interaction of a gamma ray within a continuous scintillation crystal, using either kind of PM. A current sensitive preamplifier stage with individual gain adjustment interfaces the multi-anode PM outputs with a current division resistor network. The preamplifier stage allows to improve front-end processing delay and temporal resolution behavior as well as to increase impact position calculation resolution. Depth of interaction (DOI) is calculated from the width of the scintillation light distribution, which is related to the sum of voltages in resistor network input nodes. This operation is done by means of a high-speed current mode scheme.

Calibration of a PEM detector with depth of interaction measurement

We present an in situ calibration technique for the LBNL positron emission mammography (PEM) detector module that is capable of measuring depth of interaction (DOI). The detector module consists of 64 LSO crystals coupled on one end to a single photomultiplier tube (PMT) and on the opposite end to a 64 pixel array of silicon photodiodes (PD). The PMT provides an accurate timing pulse, the PDs identify the crystal of interaction, the sum provides a total energy signal and the /spl Gamma/=PD/(PD+PMT) ratio determines the depth of interaction. We calibrate using the /sup 176/Lu natural background radiation of the LSO crystals. We determine the relative gain (K) of the PMT and PD by minimizing the asymmetry of the /spl Gamma/ distribution. We determine the depth dependence from the width of the /spl Gamma/ distribution with optimal K. The performance of calibrated detector modules is evaluated by averaging results from 12 modules. The energy resolution is a function of depth ranging from 24% FWHM at the PD end to 51% FWHM at the PMT end, and the DOI resolution ranges from 6 mm FWHM at the PD end to 11 mm FWHM at the PMT end.

Optimization of PET system design for lesion detection

Traditionally, the figures of merit used in designing a positron emission tomography (PET) scanner are spatial resolution, noise equivalent count rate, noise equivalent sensitivity, etc. These measures, however, do not directly reflect the lesion detectability using the PET scanner. Here the author proposes to optimize PET scanner design directly for lesion detection. The signal-to-noise ratio (SNR) of lesion detection can be easily computed using the theoretical expressions that the author has previously derived. Because no time-consuming Monte Carlo simulation is needed, the theoretical expressions allow evaluation of a large range of parameters. The PET system parameters can then be chosen to achieve the maximum SNR for lesion detection. The simulation study shown in this paper was focused on a single ring PET scanner without depth of interaction measurement. It can be extended to multiring (two- or three-dimensional) PET scanners and detectors with depth of interaction measurement.

A study of depth of interaction measurement using bent optical fibers [in PET scanner

Depth-of-interaction (DOI) measurement is an important capability for a detector suitable for use in a small PET scanner designed for breast imaging or small animal imaging. We have investigated a different scheme to determine DOI by using a position-sensitive photomultiplier tube (PS-PMT) at one end of an LSO (Lu/sub 2/SiO/sub 5/) crystal array for positioning and timing, and a single-channel PMT (SC-PMT) connected to the other end through bent optical fibers, with the ratio of the two signals being used to determine the DOI. The main advantages of this approach are that both the PS-PMT and the SC-PMT can provide high signal-to-noise ratio (SNR) outputs (improving DOI resolution), and that the necessary technology is available with easy readout electronics. Therefore, the design permits a relatively simple and practical solution to measure DOI while still maintaining good detector performance. We have successfully bent optical fibers, with a light loss due to fiber bending of less than 12% for bending angles from 90/spl deg/ to 130/spl deg/ and for a radius of curvature from 2 to 10 mm. Preliminary measurements for a detector with 2/spl times/2/spl times/21 mm/sup 3/ LSO scintillators and a Hamamatsu R5900-C8 PS-PMT reveal good DOI measurement capability with this design, with the measured DOI resolution ranging from 4.2 to 6 mm. It also provides excellent crystal identification and coincidence timing performance. The attenuation and scattering effects introduced by using optical fibers were also studied.

Design of a high-resolution, high-sensitivity PET camera for human brains and small animals

The authors present design parameters for a 3-D PET camera with high sensitivity (35 cm detector ring diameter, 15 cm axial field of view) and isotropic high resolution provided by detector modules capable of depth of interaction (DOI) measurement. Detector modules are made of LSO crystals (3 mm square by 30 mm deep)-the small module size and short decay time of LSO reduce the detector dead time by a factor of 14 compared to conventional EGO detector modules and narrow the coincidence window width to 4 ns. This yields an expected peak noise equivalent count rate of 800 kcps and noise equivalent sensitivity of 1370 kcps//spl mu/Ci/cc with a 20 cm diameter phantom-three to five times higher than conventional scanners. With 5 mm fwhm DOI resolution, the expected reconstructed spatial resolution is <3.0 mm fwhm throughout the entire field of view. Depth of interaction measurement information is incorporated into the reconstruction algorithm by rebinning onto a regularly spaced grid. Attenuation correction is performed with an orbiting single transmission source.